U.S. Pat. No. 4,011,522 as one example, discloses a radial flow laser in which low pressure, high velocity flows are used to form lasing zones of useful length for lasing species having rapid relaxation characteristics. With such lasers, high pumping requirements resulting from the low pressures and high velocities introduce significant weight disadvantages. These weight penalties can be minimized if a diffuser is provided to recover a significant portion of the pressure of the high Mach number, low static pressure flow. The exhaust pumping requirement may then be reduced.
The design of adequate supersonic diffusers is difficult, however, due to the fact, among others, that the Reynolds numbers associated with the lasing species of the supersonic laser are normally low. One prior approach for diffusing radially expanding supersonic flows has been to provide a multiplicity of linear diffusers located circumferentially around the cylindrical nozzle of the cylindrical laser. U.S. Pat. No. 3,998,393 and the references cited therein may be consulted as examples of such conventional linear diffusers which depend on boundary layer growth and probably separation, to create multiple oblique and Lambda shocks which decelerate the supersonic flow to a subsonic Mach number.
In a conventional prior art cylindrical laser, a large number of conventional linear diffusers are arranged around the central cylindrical axis of symmetry like the spokes of a wheel at approximately 4 or 5 degree spacings. To be effective, such conventional linear diffusers require a channel length to height ratio of the order of 10 to 1, making the employment of from 90 to 100 vanes necessary in order to reduce the diffuser diameter to an acceptable size. As a result, the half angle of the leading edges of the vanes is on the order of two degrees or less.
Disadvantages of the above prior art design have become self-evident. First, the sharp leading edges of the vanes cause cooling difficulties. Coolant passages within the vanes must typically be spaced a considerable distance from the sharp leading edge where the temperature is at its greatest. Second, the channel length to height ratio of about 10 in most applications requires vanes of such lengths that the available radial distance is largely used up so as to leave little or no room for further pressure recovery through subsequent subsonic diffusion. Typical diffusers currently being proposed for cylindrical lasers require a ratio of outside diameter to nozzle diameter of approximately 5 to 1 and achieve an 80% (or less) normal shock recovery. And, finally, prior art designs have typically resulted in large, heavy and generally inefficient devices.